fix(obs): align metrics schema and collector contracts (#4476)

fix(obs): align schema and collector contracts

Refs rustfs/backlog#1005

- align obs schema descriptors with emitted labels and metric types
- fix bucket traffic help text and TTFB bucket descriptor semantics
- add regression tests for drive, network host, bucket, and node bucket contracts

Co-authored-by: heihutu <heihutu@gmail.com>

crates/obs/src/metrics/schema/node_bucket.rs

@@ -14,7 +14,7 @@
 
 #![allow(dead_code)]
 
-use crate::{MetricDescriptor, MetricName, new_gauge_md, subsystems};
+use crate::{MetricDescriptor, MetricName, MetricSubsystem, new_gauge_md};
 use std::sync::LazyLock;
 
 const BUCKET_LABEL: &str = "bucket";
@@ -25,7 +25,7 @@ pub static BUCKET_USAGE_BYTES_MD: LazyLock<MetricDescriptor> = LazyLock::new(||
         MetricName::Custom("usage_bytes".to_string()),
         "Total bytes used by the bucket",
         &[BUCKET_LABEL],
-        subsystems::BUCKET_API,
+        MetricSubsystem::new("/bucket/usage"),
     )
 });
 
@@ -35,7 +35,7 @@ pub static BUCKET_OBJECTS_TOTAL_MD: LazyLock<MetricDescriptor> = LazyLock::new(|
         MetricName::Custom("objects_total".to_string()),
         "Total number of objects in the bucket",
         &[BUCKET_LABEL],
-        subsystems::BUCKET_API,
+        MetricSubsystem::new("/bucket/usage"),
     )
 });
 
@@ -45,6 +45,18 @@ pub static BUCKET_QUOTA_BYTES_MD: LazyLock<MetricDescriptor> = LazyLock::new(||
         MetricName::Custom("quota_bytes".to_string()),
         "Quota limit in bytes for the bucket",
         &[BUCKET_LABEL],
-        subsystems::BUCKET_API,
+        MetricSubsystem::new("/bucket/usage"),
     )
 });
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn bucket_usage_descriptors_use_bucket_usage_subsystem() {
+        assert_eq!(BUCKET_USAGE_BYTES_MD.subsystem.path(), "/bucket/usage");
+        assert_eq!(BUCKET_OBJECTS_TOTAL_MD.subsystem.path(), "/bucket/usage");
+        assert_eq!(BUCKET_QUOTA_BYTES_MD.subsystem.path(), "/bucket/usage");
+    }
+}